1. Field of the Invention
The invention relates to a tool grinding machine, and, in particular, to a spindle for a collet of a tool grinding machine.
2. Description of Relevant Art
Tool grinding machines usually have a collet for clamping an at least substantially cylindrical workpiece of the later tool. Typical examples of such tools manufactured by grinding are drills and milling cutters.
To machine the workpiece from all sides, it is rotated around the cylinder axis during machining. Ideally, the rotational axis and the longitudinal axis of the workpiece are identical in the mathematical sense. In practice, however, there are tolerances for many reasons. For example, the repeatability in clamping the workpiece is limited. Also bearing tolerances of the spindle and machining forces acting on the workpiece reduce the precision of the finished tools. However, the precision requirements for drills or milling cutters are in a range of few microns. Therefore, the workpiece is generally supported on one or more steady rests, in order to prevent a deflection of the workpiece during machining.
In EP 1419852 A1, a tool grinding machine with a spindle for a collet is described. The collet is located at the head end of the spindle, which is rotatably supported by two hydrostatic bearings opposite a bearing block. The workpiece is received by the collet and is additionally supported via a steady rest as a static bearing. The hydrostatic bearings replace the usual ball bearings. The hydrostatic bearing facing towards the workpiece allows a larger radial clearance than the hydrostatic bearing facing away from the workpiece; thereby an over-determined bearing should be avoided and inaccuracies in concentricity should be compensated. A lateral deflection of the spindle should be avoided by a correspondingly high pressure in the hydrostatic bearings.
In DE 10 2005 007 038 A1, a workpiece spindle stock for a tool grinding machine is described. The workpiece spindle stock has as usual a spindle with a collet to receive the workpiece. In order compensate inaccuracies in clamping, the so-called eccentricity of the workpiece is measured and corrected after each clamping action. For the correction, the spindle has a releasable alignment interface, which allows a motorized alignment of the collet and thus of the workpiece orthogonally to the spindle axis.
In DD 2 40 157 A1, a spindle of a machine tool is described. The spindle has a drive shaft and a working spindle. The drive shaft and the work spindle are coupled via a flexible membrane disc as rotational coupling. Machining forces occurring in axial direction are absorbed by angular contact ball bearings. The workpiece-side angular contact ball bearing is configured as a fixed bearing and the drive shaft side angular contact ball bearing allows wobble compensation.
In DE 10 2009 031 027 A1, a split tool spindle for a combined milling and turning machine with a stationary and a rotating tool is described. The tool spindle has a clamping head with a spindle shaft which is connected via a coupling to the shaft of a drive motor. For milling, the tool spindle is fixed hydrostatically in the collet.